1. Field of the Invention
The present invention relates generally to the plates employed in a viscous coupling apparatus of the type used in motor vehicles and, more particularly, to a method for manufacturing a viscous coupling plate exhibiting improved wear resistance and improved resistance to permanent deformation under operating conditions.
2. Discussion
Viscous couplings are used in various applications within the drivetrain of a motor vehicle. In general, viscous couplings include a rotatable input member and a rotatable output member, both members being rotatable about a common axis. Typically, the output member is a hub and the input member is a housing that creates a hermetically sealed chamber around the hub. Contained within the chamber are two sets of viscous coupling plates that are interleaved with one another. The first set of plates, the inner viscous coupling plates, are splined to the hub and rotatable with the hub. Similarly, a second set of plates, the outer viscous coupling plates, are splined to the housing and rotatable with the housing. Filling the remaining space within the chamber is a high viscosity fluid, such as silicone oil, and a small amount of a gas or air.
In operation, a slight difference in the rotational speed between the input and output members of the viscous coupling (i.e. slip) is permitted due to the relatively low viscous shear rate within the fluid. However, as the speed differential between the input and output members increases, the viscous shear rate of the fluid also increases. As this occurs, the fluid works to transmit torque from the input member to the output member. Eventually, a pressure increase occurs within the chamber and forces adjacent inner and outer plates into contact with one another, resulting in torque being transferred by friction between the mating plates in addition to the viscous shear rate of the fluid. Once frictional contact has been established between the plates, torque transmission is suddenly and significantly amplified.
Viscous coupling plates are generally manufactured from a thin plate stock, such as a carbon steel plate material, that is formed by a series of dies in a stamping operation of a machine press. Subsequent to forming operations, the viscous coupling plates are subjected to heat treatment processes in an effort to achieve the desired mechanical and physical properties, such as strength, hardness, toughness and wear resistance, that are required in the finished product.
However, it has been learned through experience that conventional heat treatment techniques create distortion in the shape of the viscous coupling plates, causing the plates to be wavy and to fail to meet established requirements for flatness. Further, it is believed that heat treatment generally tends to create a non-uniform distribution of any residual stresses that remain in the plate as a result of the manufacturing operations which formed the plate. The combination of these heat treatment side effects causes the plates to be susceptible to "dishing", a phenomenon of permanent deformation that results from the operating conditions that are experienced by the plates in the viscous coupling apparatus. As a consequence of dishing, the performance of the viscous coupling apparatus is seriously degraded.
Previous attempts have been made to solve the problem of distortion that occurs in thin flat workpieces during heat treatment. Generally, these prior efforts have concentrated on methods involving subjecting a plurality of the distorted workpieces to a second heat treatment while the workpieces are placed under a compression force. Thus, each workpiece, having already undergone a first heat treatment to enhance its material properties, is processed in a second heat treatment that is aimed at reducing or eliminating the distortion. In the subsequent heat treatment, a plurality of the workpieces are fixtured so as to be stacked upon one another and clamped together under a compression force. These prior methods to reduce distortion in thin, flat workpieces, however, require strict controls on the second heat treatment process or result in negating the positive effects of the first heat treatment.
It is therefore an object of the present invention to overcome the disadvantages associated with conventional viscous coupling plates and to provide a method for manufacturing a viscous coupling plate which exhibits resistance to wear and to permanent deformation under operating conditions.
It is another object of the present invention to provide an improved method for hardening and flattening a viscous coupling plate. It is also an object of this invention is to produce a viscous coupling plate which exhibits the above advantages and which is cost effective to manufacture in a high volume production environment.
In achieving the above objects, the present invention provides for a method of hardening and flattening a viscous coupling plate generally combining a heat treatment process followed by a mechanical forming operation. The heat treatment process of the present invention generally comprises the steps of first surface hardening a viscous coupling plate by ferritic nitrocarburizing to produce an epsilon iron nitride surface layer on the plate. The plates are then quenched. Next, the plate is tempered without requiring that several plates be clamped together or otherwise touching during this step. Finally, each plate is worked in a mechanical forming operation comprising a flattening process. This is contrary to the accepted teachings in the prior art which discount the effectiveness of such individualized, workpiece-flattening efforts. The flattening process comprises passing the plates through a series of leveling rolls which cause the plates to flex in opposite directions to produce a sufficiently flat plate. The flattening process can incorporate a single pass or multiple passes through the leveling rolls. The flattening process eliminates any distortion in the plates and brings them within any required flatness specifications. Also, the flat rolling process serves to more uniformly redistribute any residual stresses that remain in the plates.
The resulting viscous coupling plates exhibit superior performance characteristics than prior plates. Most notably, the plates of the present invention resist the permanent deformation that has been experienced by prior viscous coupling plates under identical operating conditions.
Various other features and advantages of the present invention will become apparent to those skilled in the art to which this invention relates after having the benefit of studying the teachings of the specification and the claims.